Biomass is a low cost readily available energy source that is useful for the generation of other energy types such as electricity or liquid fuel. Biomass derived energy has the added advantage that it is intrinsically carbon neutral and can also be carbon negative when residual processed carbon is placed in the ground for long term storage.
A popular intermediate product of biomass energy conversion is synthesis gas, which is produced by the high temperature partial combustion of the material. Devices which support the gasification of biomass are typically called gasifiers. Synthesis gas often includes the nitrogen from the source air supply, carbon monoxide, carbon dioxide, water vapor, and hydrogen. The carbon monoxide and hydrogen components are combustible and can be used in internal combustion engines to turn generators to make electricity.
Hydrogen is generally preferred as the principle combustion gas because of its high flame speed and very wide combustion range. However, hydrogen production typically involves the water shift reaction which requires high temperature and absorbs substantial reaction energy. A high concentration of carbon monoxide can be produced from gasifiers running at lower temperatures such as due to high material flow through the gasification-partial combustion region. This may result in higher thermal conversion efficiency from biomass to combustible gas, however, carbon monoxide rich synthesis gas is generally more difficult to burn in an internal combustion engine than a hydrogen rich gas.
Internal combustion engine efficiency is dominated by the Carnot Cycle which specifies that the maximum thermal efficiency is dictated by the engine compression ratio. For example, an air throttled spark ignition engine may have a maximum theoretical efficiency of only 35% at part load whereas a high compression diesel, at a 20:1 compression ratio has a limiting efficiency of 70%. In practice, engine frictional loads, accessories and system heat losses reduce real world engines to about half of their Carnot limit. Generally, diesels are more efficient than spark ignition engines. Diesels rely on high cetane fuels which can ignite by the high pressure alone in a diesel as the piston approaches the top of its rotational arc—(Top Dead Center or TDC). Spark ignition engines, at a lower compression ratio, rely on high octane fuels. Such fuels require a high energy spark to ignite and generally burn more slowly than high cetane diesel fuels when subjected to high pressures. Both carbon monoxide and hydrogen are rated as high octane fuels, but hydrogen has an unusually high flame speed because of its small molecular size and high reaction energy.